As disclosed in Non-patent Document 1, with the aim of improving mileage by reducing a load applied to an engine in a commercial vehicle such as a truck, there has recently been proposed a parallel hybrid system using an assisting power of a motor during driving where a load is applied to the engine such as during start-up or during uphill driving.
As illustrated in FIG. 6, the arrangement of a driving system of the above hybrid system is such that an engine 1, a main clutch (single plate clutch) 3, a motor (motor/generator) 5, a transmission 7, and a final reduction gear 9 are all arranged coaxially in the order mentioned. A rotor 13 of the motor 5 is attached to an engine drive shaft system 11 extending from the main clutch 3 to the transmission 7, and a propeller shaft 15 is coupled between the transmission 7 and the final reduction gear 9.
With this structure, the aforesaid hybrid system starts only with a driving force of the motor 5 with the clutch 3 being disconnected, and when the vehicle reaches a prescribed speed (in the vicinity of about 5 to 6 km/h), it synchronizes the rotations of the engine 1 and the motor 5, connects the clutch 3, and starts driving only with the engine 1.
Then, during the driving where a load is applied, such as during uphill driving, the system enters high-power driving where the engine 1 and the motor 5 are used in combination according to a depression amount of an accel pedal, and during deceleration/braking, according to the situation, the clutch 3 is disconnected and the motor 5 is reversely operated, whereby a regenerative brake corresponding to an engine brake is applied, and a regenerative energy is stored in batteries.
However, a power transmission mechanism of the aforesaid hybrid system has a drawback that during the driving only with the driving force of the engine 1, a mechanical loss occurs due to an influence of a magnet put on the rotor 13. Further, supplying electricity to a stator 17 of the motor 5 in order to cancel such a load loss newly causes the occurrence of an electric loss.
As a power transmission mechanism of a hybrid system realizing a reduction of such a loss, Non-patent Document 2 discloses an art in which a motor output shaft 19 is structured separately from an engine drive shaft system 11 and a clutch mechanism 21 formed by a dog clutch is mounted between the motor output shaft 19 and the engine drive shaft system 11, as illustrated in FIG. 7.